The Devonian was a time of wonder and mystique. The Age of Fishes, it capped the rise of vertebrates and heralded the rise of skeletal diversity. Fish in this age began to inch towards the shore; some would have crawled through the muck of it; some others still out of the water.

But before the Carboniferous rolled in, with its skeleton-enhanced fish, giant insects, and great plants, the Devonian paid heed to a relatively diverse group of “fish,” the placoderms, by giving us the “hinge necked” fish, the Arthrodira. Placoderms are relatively simple looking, with a generally scaly body, simple fins, and a large head fashioned into a sort of shield. The mouth has become enhanced with a bony core for the mandible, otherwise a fleshy-flap within which lay a cartilage framework, and its margin was developed into a variety of shapes, including prongs, saws, or a relatively smooth edge. But amongst the greatest of arthrodires was Dunkelosteus which, along with other giants like Titanichthys, Dinichthys, Eastmannosteus, formed a cadre of super fishes, most of whom were predators (Titanichthys being considered a filter feeder; see Janvier, 1998). Their jaws arranged into an elaborate system of sawing edges, their maws must have been utterly terrifying had we, us wee unarmored humans, ever seen them in the flesh.

Dunkelosteus, from Steve White. Shared with permission.

Those jaws are not apparently for show: Some research by Philip Anderson and Mark Westneat on one species (Dunkleosteus terrelli, the largest) indicated the ability to produce bites of between 4-5k Newtons of force, which is small in relation to, say, white sharks (Carcharias sp.) at 20k, or the Hell Creek tyrant (Tyrannosaurus rex) at over 35k Newtons (with work by Bates and Falkingham). When you consider even the Newton-to-weight ratio, this is pretty high: Dunkleosteus is smaller than White Sharks, but had a comparable bite; and though much smaller bite than in tyrants was only about 1/10th as strong. So chock one up for tyrants: He needs more publicity.

But that’s nothing compared to piranhas. Newtons to weight, piranhas have a much stronger bite: merely inches long, but producing upwards of 300 N in Piaractus brachypomus, the giant Megapiranha (totally not a made up name) paranensis had a bite of 4k N, but was “only” 70cm long (that’s almost two feet for use ‘Merkans; this revises an earlier, 1m estimate). This study from Justin Grubich of the Egyptian Dept. of Biology at American University in Cairo and colleagues took the bite forces of a variety of fishes and scaled them by their size, with clear outliers being the tiny fish who gave Teddy Roosevelt a second look.

Image from Cracked.com. (Note: estimate of size is over-large, and a downward 0.7m length is more likely.)

But here’s the thing: When a white shark or a piranha closes its mouth, the teeth disappear. They are enveloped in soft-tissue, the formation of which preceded the bony jaws within them. Arthrodires have jaw bones with shearing edges, bone that has been scraped against bone, resembling a butcher’s blade stuck to the end of each jaw and then rasped against another set. Giant butcher’s cleavers scraping in the Devonian seas, making a horrible shrp shrrr shrp shrrr sound. They ground against one another, forming an eversharpening edge. The edges are curved, too, which would have helps focus force to the spaces between the cusps, the “biting teeth” of the jaw’s margin, the teeth in the saw.

Certainly, some part of the jaws wouldn’t be embedded in fleshy stuff, but certainly not all of it, exposing the mandibles to full view. We’re not talking bare bones here; some soft tissue much be present around the jaws. The question is how much? and would the horrifying cleavers from some stygian abyss been visible as the jaw was closed? Or, instead, would we get this:

Probably an unfair comparison, but it certainly softens the image, somewhat. Makes it look more like a beluga, less like a skeletal horror from beyond. This follows comparison to living fish:

The pliability of the oral integument differs from piranha species to piranha species, and this one (Pygocentrus nattereri) is an especially “lippy” one, whilst some others are far thinner in the tissue. But when all piranhas close their mouths, the teeth disappear. The traditional method of restoring arthrodires, amongst other placoderms, has been to leave the head in a sort of thin shroud of tissue, and this may be true of some fish, but around the oral margin things differ, so thinking about this leads me to suspect a stranger look. One that is, perhaps, fishier, and less frightsome.

15 Responses to A Saw in the Jaw of a Sea-God

When I used to have pet piranhas, you couldn’t easily see their teeth. But when they’d do a yawn… that was another story! They would periodically (like the theropods) shed their teeth… which were replaced by new ones; one would often see the shed teeth in the gravel below. I used to have to catch the piranhas (for occasional transport to another tank) with a metal kitchen colander; they would instantly eat right through regular large nets! They could shred the thick plastic plants in the aquariums just as if they were made of butter. I can’t imagine the power of Megapiranha! ;)

Yeah, the little guys have such an awesome reputation, and prove this out often enough, even if the Teddy-based myth is a little far-fetched. Megapiranha would certainly be a little awesome terror in its ecology.

Awesome to see a post with some arthrodires in it. Most people have no idea what they are! Just a quick note, Titanichthys has no “teeth” along its jaw and instead was probably the Devonian equivalent of a basking shark. A great filter-feeder of the past. I’m actually working on descriptive paper of a Titanichthys specimen from Ohio that I will hopefully be submitting in the coming weeks.

Some good points here. A similar dilemma as the theropod “lizard or crock lips”. Until we find the firm material evidence, it’ll all stay in the domain of speculation…. The hunch, feeling and personal preference in restoring. Frankly, I don’t know which side to take.
Your Dunkleosteus with lips does make sense, though.
Currently, regarding the dinosaur lips I am biased towards the keratinous lips. Before some discoveries were published, I was a strong advocate of lizard lips.

I would have to elaborate that, but here is the big dilemma in a few words: It was notorious that “many” dinosaurs and all of the birds had keratinous beaks. In fact, all except sauropods and (presumably) most of the theropods ( minus the birds?) Right? Why the exception? However, recently it became obvious there were also some (weird?) sauropods and theropods with indisputable beaks or keratinous, fused lips. Only some, or all of them? The dinosaur skulls were built to be very strong and very light. The perfect solution was using the keratin. The keratinous beak can be combined with teeth, as we can see in some dinosaurs and all the crocks (even the non-amphibious ones?) and turtles…. Why not in all of the dinosaurs? The keratin can only make the jaws stronger.http://www.pnas.org/content/early/2013/11/27/1310711110.full.pdf

“It was notorious that “many” dinosaurs and all of the birds had keratinous beaks.”

Not necessarily. It was Greg Paul who argued that most theropods had a sort of beaky “remnant” on their jaws because, he thought, many theropod lineages were descendants of a bird-dromie split, and were on the dromie line. That meant, for him, oviraptorids, troodonts, avimimids, caenagnathids, dromaeosaurids, and a sundry other few were bird-descendants of a sort. Unfortunately, he based this on his impression that Archaeopteryx lithogaphica had a beak, a conclusion based solely on the shape of the rostrum and nothing to do with the gross morphology of the jaw. In fact, Greg’s argument that some physical features related to the beak (numerous foramina, irregular pattern of the foramina, and even a more “roughened” texture of the bone) doesn’t even apply to Archaeopteryx. We now know that a smoother bone texture, smaller and numerous, not just large and irregular, foramina pattern are better predictors of beak position (Morhardt’s thesis does some work on this, and it was proposed in print by Keillor (2013) and has been the subject of research by Tobin Hieronymus and the Witmer Lab).

“In fact, all except sauropods and (presumably) most of the theropods ( minus the birds?) Right?”

There’s no reason to exclude sauropods from this exception. In some ways, they display better analogies to Greg’s argument for theropod beaks than do many theropods (rough facial surfaces, long grooved regions of the face, numerous small foramina). There’s just an abject refusal to think of anything but an ornithischian or bird-line theropod with a beak. Of course, this doesn’t mean they had beaks; I don’t think they did, nor do I think all but a small handful of theropods did.

“Why the exception?”

For a while, sauropods had trunks. Sauropod paleobiology never seemed to suggest to people “beaks” would be present. For one, they had a long row of teeth, while at the same time they were sluggish, plat-feeding behemoths that had to spend all their time in the water. Ornithischians were land-dwelling, had no teeth at the front of their jaws, and their beaks it seemed were obviously covered in keratin. We’ve also now found traces of this in some ankylosaurs and hadrosaurs, and so they were never speculated to NOT have beaks. But theropods are different. Theropods are on the line to birds, so there must have been beaks SOMEWHERE. This opens the gates to fancy, and faulty reasoning: the type that allows them to have beaks, but not sauropods, despite the poorer relation of evidence.

“However, recently it became obvious there were also some (weird?) sauropods and theropods with indisputable beaks or keratinous, fused lips.”

You speak of Bonitasaura salgadoi. Yes, it was proposed that some nemegtosaurid titanosaurs would have had a region of their jaws (between teeth and adductor muscles, which were reconstructed very sparsely) with a keratin sheath on them. But this theory was more recently knocked down for the region being comaprable to the position of the coronoid elements of the mandible (Wilson, 2007), which in Nemegtosaurus mongoliensis form the thin lamina that Apesteguia (2004) considered evidence of a sheath. However, more recently than that Gallina & Apesteguia (2011) have supported the earliest claim with relation of the irregular, “rough” texture, though without foramina) as being comparable ti bovine horns and birds’ beaks. It is uncertain, for no histological work has been done, and none of the authors have argued how these structures may not relate to the enthesial attachments of ligaments to the jaw, for the muscles. A lamina doesn’t necessarily have to be a beak margin, and such do occur when a bone lies between two muscles. This is the base condition for the medial dentary bones, after all. It should be pointed out that a thin lamina has also been used as the predictor for the presence of a “cheek,” such that its presence in many ornithischian dinosaurs as well as therizinosauroids meant that some soft-tissue is present. Czerkas’s argument that this lamina supported a beak instead in Stegosaurus is recursive back to Apesteguia’s argument, but it hasn’t been necessarily supported.

“The dinosaur skulls were built to be very strong and very light. The perfect solution was using the keratin. The keratinous beak can be combined with teeth, as we can see in some dinosaurs and all the crocks (even the non-amphibious ones?) and turtles…. Why not in all of the dinosaurs? The keratin can only make the jaws stronger.”

This is all true. As I mentioned when I discussed Lautenschlager et al.’s recent work (here, refs within) a beak does help reduce stress within the jaw, but one can have too much of this, as it doesn’t confer a positive effect after a certain point. It should also be noted that cranial flexibility is important in many ways for the dissipation of forces on the jaws; an open symphysis is useful in allowing forces to be separated from units of the skull, whereas interfingering or fusion of these symphyses lock some bones together, allowing forces to “take a course” through units of the skull. This is why only some tyrannosaurid skull bones are fused, while others are firmly locked together, their contacts extensively interfingering (class III symphysis; lacrimal to nasal to maxilla, nasal to frontal) or fused (class IV symphysis; frontal to parietal to supraoccipital).

Another example, as related by Holliday and Nesbitt (see here for more details) is the mandibular symphysis, in which only animals that are resisting high forces at the jaw require interfingering or fusion, and that this can be a predictor of types of loads on the jaw. Hypothetically, if a symphysis isn’t fused (as in therizinosaur mandibles) then one can assume the forces on the joint aren’t that high. And that’s what we see in Erlikosaurus jaws: the symphysis is open because the forces aren’t particularly high; yet when a keratinous beak is added, force is dissipated and what forces on the symphysis are present when biting at the tip are reduced, yet expanding the beak doesn’t change much. We’d expect the same things for other taxa. This is thus a useful method to examine skulls (like some sauropods) to assume whether a cornified sheet of skin (or “beak”) might be present, if its presence helps dissipate bite forces in ways other types of biting (or other actions of the head) wouldn’t. Thus, likelihood is that pachycephalosaur domes had these sheets; in Erlikosaurus, however, the beak only extends around the areas where there are no teeth. Because teeth perform similar stress reduction as beaks do, while costlier but lighter, their presence makes a fair predictor of whether beaks are present.

Thus, where there are teeth, beaks (or other forms of keratin enforcement) may not necessarily be. Thickened, cornified skin, as in the faces of crocs, may be present to dissipate forces of the underlying skeleton away from the skull itself, which would help further reduce bite stresses that would further imperil the skull’s integrity. Such features seem to be present in tyrannosaurids, and might be present in other heavy-biting theropods, but our arguments for biting in sauropods is that they had much, much weaker bites: there’d be little cause to claim they needed such skin, even if it would be useful. (as a note, cornified skin doesn’t always have to be about stress reduction, as its presence in bird faces — this is what cere is — has been adapted for display purposes.

An excellent elaboration, Jaime! Still there are too many “ifs”. I’ve learned to become both inquisitive and conservative regarding the general trends in reconstructing and restoring prehistoric animals.

There are always “ifs,” be we can make strong inferences based on known biological allowances and limitations. By arguing that extinct organisms obey the same rules and principles of physics, biology and development, so we don’t invent things for our fancies (other than propositional hypotheses which we test), we find ourselves with a limited number of answers which to draw from. They limit how wild our ideas can become.

Can’t argue with that. What makes paleontology discoveries interesting is that they never stop to amaze us, although, most of them had been and are, in fact, expected. Tell me the true: Where you really surprised by the discovery of the fuzzy giant Yutyrannus? I was more amused about the fact. Actually, Bob’s revolutionary illustration of Deinonychus made less impact on me when I first saw it (1969/70?), than McLoughlin’s “Archosauria” a few years later. While Bakker’s agile dino still looked a lot like a lizard, in his bold book McLoughlin made a really strong statement with his fuzzy little theropods and dinosaurs with cheeks. It changed my view on dinosaurs instantly and revived my interests at the same time.